OK - I am in an unique position he I actually have worked with true 2-phase power, developed in the 1920s before 3-phase was well-established, as a means to provide off-set to start motors. Also pretty much confined to Philadelphia and Baltimore, being the two major cities in what became the PMJ Interconnect.

From PECO Tariffs:

Two-phase power is where the two phases are 90° apart.

This is a four (4) wire system, and the neutral currents do not cancel even if the system is in balance. Hence the need for four (4) wires.

I am surprised that so many went after the remark of audio and pacemakers. But here goes:

Pacemakers will accept all sorts of RF and other interference today - a vast improvement from the days when merely walking past a vintage microwave (in operation) would cause troubles.

But the modern pacemaker/defibrillators do not like stray currents in the body, as they may be taken as an event. If there is as much as a few volts difference between the NEUTRAL and the GROUND, and an individual so-equipped steps into that difference, that could be enough to trip the defib-function. Not (usually)fatal, but quite painful. Just ask the guy up on the 10th floor designing temporary artificial hearts - between restoring vintage Porsches. He will talk the paint off a board if given a chance - and I am sufficiently intrigued by what he does to give him those chances.

And, of course, there are hum-loops caused by stray currents.

Peter Wieck
Melrose Park, PA